Out-of-Equilibrium Colloidal Assembly Driven by Chemical Reaction Networks

Enzymatically-induced dynamic assemblies from surface functional stomatocyte nanoreactors

Collective behavior has become a recent topic of investigation in systems chemistry. In pursuing this phenomenon, we present polymersome stomatocytes loaded with the enzyme urease, which show basic stigmergy-based communication and are capable of signal production, reception, and response by clustering with surface complementary binding partners. The collective behavior is transient and based on the widely known pH-sensitive non-covalent interactions between nitrilotriacetic acid (NTA) and histidine (His) moieties attached to the surface of urease-loaded and empty stomacytes, respectively. Upon the addition of the substrate urea, the urease stomatocytes are able to increase the environmental pH, allowing the NTA units to interact with the surface histidines on the complementary species, triggering the formation of transient clusters. The stomatocytes display a maximum clustering interaction at a pH between 6.3 and 7.3, and interparticle repulsive behavior outside this range. This leads to oscillating behavior, as the aggregates disassemble when the pH increases due to high local urease activity. After bulk pH conditions are restored, clustering can take place again. Within the detectable region of dynamic light scattering, individual stomatocytes can aggregate to agglomerates with 10 times their volume. Understanding and designing population behavior of active colloids can facilitate the execution of cooperative tasks, which are not feasible for individual colloids.

Programmable Crowding and Tunable Phases in a Binary Mixture of Colloidal Particles under Light-Driven Thermal Convection

We employ photothermally driven self-assembly of colloidal particles to design microscopic structures with programmable size and tunable order. The experimental system is based on a binary mixture of "plasmonic heater" gold nanoparticles and "assembly building block" microparticles. Photothermal heating of the gold nanoparticles under visible light causes a natural convection flow that efficiently assembles the microscale building block particles (diameter 1-10 μm) into a monolayer. We identify the onset of active Brownian motion of colloidal particles under this convective flow by varying the conditions of light intensity, gold nanoparticle concentration, and sample height. We realize a crowded assembly of microparticles around the center of illumination and show that the size of the particle crowd can be programmed using patterned light illumination. In a binary mixture of gold nanoparticles and polystyrene microparticles, we demonstrate the formation of rapid and large-scale crystalline monolayers, covering an area of 0.88 mm2 within 10 min. We find that the structural order of the assembly can be tuned by varying the surface charge of the nanoparticles and the size of the microparticles, giving rise to the formation of different phases-colloidal crystals, crowds, and gels. Using Monte Carlo simulations, we explain how the phases emerge from the interplay between hydrodynamic and electrostatic interactions, as well as the assembly kinetics. Our study demonstrates the promise of self-assembly with programmable shapes and structural order under nonequilibrium conditions using an accessible setup comprising only binary mixtures and LED light.

Theory and simulations of light-induced self-assembly in colloids with quantum chemistry derived empirical potentials

Light-induced self-assembly (LISA) is a non-invasive method for tuning material properties. Photoresponsive ligands coated on the surfaces of nanoparticles are often used to achieve LISA. We report simulation studies for a photoresponsive ligand, azobenzene dithiol (ADT), which switches from a trans-to-cis configuration on exposure to ultraviolet light, allowing self-assembly in ADT-coated gold nanoparticles (NPs). This is attributed to a higher dipole moment of cis-ADT over trans-ADT which leads to a dipole-dipole attraction facilitating self-assembly. Singh and Jha [Comput. Theor. Chem., 2021, 1206, 113492] used quantum-chemistry calculations to quantify the interaction energy of a pair of ADT ligands in their cis and trans conformations. The interaction energy between ligands was fit to a potential energy function of the Lennard-Jones (LJ) form having distinct exponents for attractive and repulsive contributions. Using this generalized equation for the ligand-ligand interaction energy, we calculated the total effective interaction energy between a pair of cis as well as trans ADT-coated NPs. Specifically, we calculated the effective interaction energies between cis/trans-NPs using discrete as well as continuous approaches. Given the limitations of experiments in probing individual ligand conformations, we also studied the effect of varying the functional ligand length on the interaction energy between NPs and identified the optimal functional ligand length to capture the steric and conformational effects. Finally, using the effective interaction energy, we obtained a generalized potential energy function, which was applied in Langevin dynamics simulations to capture self-assembly in NPs.

Non-Equilibrium Dissipative Assembly with Switchable Biological Functions

Natural dissipative assembly (DSA) often exhibit energy-driven shifts in natural functions. However, creating man-made DSA that can mimic such biological activities transformation remains relatively rare. Herein, we introduce a cytomembrane-like dissipative assembly system based on chiral supramolecules. This system employs benzoyl cysteine in an out of equilibrium manner, enabling the shifts in biofunctions while minimizing material use. Specifically, aroyl-cystine derivatives primarily assemble into stable M-helix nanofibers under equilibrium conditions. These nanofibers enhance fibroblast adhesion and proliferation through stereospecific interactions with chiral cellular membranes. Upon the addition of chemical fuels, these functional nanofibers temporarily transform into non-equilibrium nanospheres, facilitating efficient drug delivery. Subsequently, these nanospheres revert to their original nanofiber state, effectively recycling the drug. The programmable function-shifting ability of this DSA establishes it as a novel, fuel-driven drug delivery vehicle. And the bioactive DSA not only addresses a gap in synthetic DSAs within biological applications but also sets the stage for innovative designs of 'living' materials.

Enzymatic Dissociation of DNA-Histone Condensates in an Electrophoretic Setting: Modulating DNA Patterning and Hydrogel Viscoelasticity

Development of an energy-driven self-assembly process is a matter of interest for understanding and mimicking diverse ranges of biological and environmental patterns in a synthetic system. In this article, first we demonstrate transient and temporally controlled self-assembly of a DNA-histone condensate where trypsin (already present in the system) hydrolyzes histone, resulting in disassembly. Upon performing this dynamic self-assembly process in a gel matrix under an electric field, we observe diverse kinds of DNA patterning across the gel matrix depending on the amount of trypsin, incubation time of the reaction mixture, and gel porosity. Notably, here, the micrometer-sized DNA-histone condensate does not move through the gel and only free DNA can pass; therefore, transport and accumulation of DNA at different zones depend on the release rate of DNA by trypsin. Furthermore, we show that the viscoelasticity of the native gel increases in the presence of DNA and a pattern over gel viscoelasticity at different zones can be achieved by tuning the amount of enzyme, i.e., the dissociation rate of the DNA-histone condensate. We believe enabling spatiotemporally controlled DNA patterning by applying an electric field will be potentially important in designing different kinds of spatiotemporally distinct dynamic materials.

Programmable Enzymatic Reaction Network in Artificial Cell-Like Polymersomes

The ability to precisely control in vitro enzymatic reactions in synthetic cells plays a crucial role in the bottom-up design of artificial cell models that can recapitulate the key cellular features and functions such as metabolism. However, integration of enzymatic reactions has been limited to bulk or microfluidic emulsions without a membrane, lacking the ability to design more sophisticated higher-order artificial cell communities for reconstituting spatiotemporal biological information at multiple length scales. Herein, droplet microfluidics is utilized to synthesize artificial cell-like polymersomes with distinct molecular permeability for spatiotemporal control of enzymatic reactions driven by external signals and fuels. The presence of a competing reverse enzymatic reaction that depletes the active substrates is shown to enable demonstration of fuel-driven formation of sub-microcompartments within polymersomes as well as realization of out-of-equilibrium systems. In addition, the different permeability characteristics of polymersome membranes are exploited to successfully construct a programmable enzymatic reaction network that mimics cellular communication within a heterogeneous cell community through selective molecular transport.

Stiffness Modulation and Pulsatile Release in Dual Responsive Hydrogels

Inspired by nature, self-regulation can be introduced in synthetic hydrogels by incorporating chemo-mechanical signals or coupled chemical reactions to maintain or adapt the material's physico-chemical properties when exposed to external triggers. In this work, we present redox and light dual stimuli responsive hydrogels capable of rapidly adapting the polymer crosslinking network while maintaining hydrogel stability. Upon irradiation with UV light, polymer hydrogels containing redox responsive disulfide crosslinks and light responsive ortho-nitrobenzyl moieties show a release of payload accompanied by adaptation of the hydrogel network towards higher stiffness due to in situ crosslinking by S-nitrosylation. Whereas the hydrogel design allows the network to either become softer in presence of reducing agent glutathione or stiffer upon UV irradiation, simultaneous application of both stimuli induces network self-regulation resulting in a pulsatile form of payload release from the hydrogel. Finally, adaptive stiffness was used to make tunable hydrogels as substrates for different cell lines.

Bio-Inspired Far-From-Equilibrium Hydrogels: Design Principles and Applications

Inspired from dynamic living systems that operate under out-of-equilibrium conditions in biology, developing supramolecular hydrogels with self-regulating and autonomously dynamic properties to further advance adaptive hydrogels with life-like behavior is important. This review presents recent progress of bio-inspired supramolecular hydrogels out-of-equilibrium. The principle of out-of-equilibrium self-assembly for creating bio-inspired hydrogels is discussed. Various design strategies have been identified, such as chemical-driven reaction cycles with feedback control and physically oscillatory systems. These strategies can be coupled with hydrogels to achieve temporal and spatial control over structural and mechanical properties as well as programmable lifetime. These studies open up huge opportunities for potential applications, such as fluidic guidance, information storage, drug delivery, actuators and more. Finally, we address the challenges ahead of us in the coming years, and future possibilities and prospects are identified.

A simple method to alter the binding specificity of DNA-coated colloids that crystallize

DNA-coated colloids can crystallize into a multitude of lattices, ranging from face-centered cubic to diamond, opening avenues to producing structures with useful photonic properties. The potential design space of DNA-coated colloids is large, but its exploration is hampered by a reliance on chemically modified DNA that is slow and expensive to commercially synthesize. Here we introduce a method to controllably tailor the sequences of DNA-coated particles by covalently appending new sequence domains onto the DNA grafted to colloidal particles. The tailored particles crystallize as readily and at the same temperature as those produced via direct chemical synthesis, making them suitable for self-assembly. Moreover, we show that particles coated with a single sequence can be converted into a variety of building blocks with differing specificities by appending different DNA sequences to them. This method will make it practical to identify optimal and complex particle sequence designs and paves the way to programming the assembly kinetics of DNA-coated colloids.

Dynamic surface chemistry and interparticle interactions mediating chemically fueled dissipative assembly of colloids

HYPOTHESIS

Dissipative assembly of colloids involves using a chemical fuel to temporarily activate organic colloid surface ligands to an assembly prone state. Colloids assemble into transient aggregates that disintegrate after the fuel is consumed. The underlying colloidal interactions controlling dissipative assembly have not been rigorously identified or quantified. We expect that fuel concentration dependent dissipative assembly behavior can be reconciled with measurements of dynamic colloid surface chemistry and colloidal interactions.

EXPERIMENTS

Carbodiimide chemistry was utilized to induce dissipative assembly of carboxylic acid functionalized polystyrene colloids. We measured aggregation kinetics, colloid surface hydrophobicity, and zeta potential as a function of time, which established that colloids underwent dissipative assembly for fuel concentrations between 5 and 12.5 mM and irreversible aggregation at higher fuel concentrations due to transient changes in surface chemistry.

FINDINGS

We formulated a pairwise colloidal interaction potential model including hydrophobic interactions quantified by fluorescence binding experiments. Fuel concentrations causing dissipative assembly displayed a transient increase in secondary minimum depth and a primary maximum much larger than the thermal potential. Fuel concentrations leading to irreversible aggregation displayed a primary maximum smaller than the thermal potential. This is the first study to quantify surface chemistry and interparticle interactions during dissipative colloid assembly and represents a foundational step in rationally designing more complex dissipative assembly systems.

Transient co-assemblies of micron-scale colloids regulated by ATP-fueled reaction networks

Self-assembly of colloidal particles offers an attractive bottom-up approach to functional materials. Current design strategies for colloidal assemblies are mostly based on thermodynamically controlled principles and lack autonomous behavior. The next advance in the properties of colloidal assemblies will come from coupling these structures to out-of-equilibrium chemical reaction networks furnishing them with autonomous and dynamic behavior. This, however, constitutes a major challenge of carefully modulating the interparticle potentials on a temporal circuit program and avoiding kinetic trapping and irreversible aggregation. Herein, we report the coupling of a fuel-driven DNA-based enzymatic reaction network (ERN) to micron-sized colloidal particles to achieve their transient co-assembly. The ERN operating on the molecular level transiently releases an Output strand which links two DNA functionalized microgel particles together into co-assemblies with a programmable assembly lifetime. The system generates minimal waste and recovers all components of the ERN after the consumption of the ATP fuel. The system can be reactivated by addition of new fuel as shown for up to three cycles. The design can be applied to organize other building blocks into hierarchical structures and materials with advanced biomimetic properties.

Temporal Self-Regulation of Mechanical Properties via Catalytic Amyloid Polymers of a Short Peptide

We report a short peptide that accessed dynamic catalytic polymers to demonstrate four-stage (sol-gel-weak gel-strong gel) temporal self-regulation of its mechanical properties. The peptide exploited its intrinsic catalytic capabilities of manipulating C-C bonds (retro-aldolase-like) that resulted in a nonlinear variation in the catalytic rate. The seven-residue sequence exploited two lysines for binding and cleaving the thermodynamically activated substrate that subsequently led to the self-regulation of the mechanical strengths of the polymerized states as a function of time and reaction progress. Interestingly, the polymerization events were modulated by the different catalytic potentials of the two terminal lysines to cleave the substrate, covalently trap the electrophilic products, and subsequently control the mechanical properties of the system.

Oppositely Charged Nanoparticles Precipitate Not Only at the Point of Overall Electroneutrality

Precipitation of oppositely charged entities is a common phenomenon in nature and laboratories. Precipitation and crystallization of oppositely charged ions are well-studied and understood processes in chemistry. However, much less is known about the precipitation properties of oppositely charged nanoparticles. Recently, it was demonstrated that oppositely charged gold nanoparticles (AuNPs), also called nanoions, decorated with positively or negatively charged thiol groups precipitate only at the point of electroneutrality of the sample (i.e., the charges on the particles are balanced). Here we demonstrate that the precipitation of oppositely AuNPs can occur not only at the point of electroneutrality. The width of the precipitation window depends on the size and concentration of the nanoparticles. This behavior can be explained by the aggregation of partially stabilized clusters reaching the critical size for their sedimentation in the gravitational field.

Reaction-driven assembly: controlling changes in membrane topology by reaction cycles

Chemical reaction cycles are prototypical examples how to drive systems out of equilibrium and introduce novel, life-like properties into soft-matter systems. We report simulations of amphiphilic molecules in aqueous solution. The molecule's head group is permanently hydrophilic, whereas the reaction cycle switches the molecule's tail from hydrophilic (precursor) to hydrophobic (amphiphile) and vice versa. The reaction cycle leads to an arrest in coalescence and results in uniform vesicle sizes that can be controlled by the reaction rate. Using a continuum description and particle-based simulation, we study the scaling of the vesicle size with the reaction rate. The chemically active vesicles are inflated by precursor, imparting tension onto the membrane and, for specific parameters, stabilize pores.

Light-Driven Self-Oscillation of Thermoplasmonic Nanocolloids

Self-oscillation-the periodic change of a system under a non-periodic stimulus-is vital for creating low-maintenance autonomous devices in soft robotics technologies. Soft composites of macroscopic dimensions are often doped with plasmonic nanoparticles to enhance energy dissipation and generate periodic response. However, while it is still unknown whether a dispersion of photonic nanocrystals may respond to light as a soft actuator, a dynamic analysis of nanocolloids self-oscillating in a liquid is also lacking. This study presents a new self-oscillator model for illuminated colloidal systems. It predicts that the surface temperature of thermoplasmonic nanoparticles and the number density of their clusters jointly oscillate at frequencies ranging from infrasonic to acoustic values. New experiments with spontaneously clustering gold nanorods, where the photothermal effect alters the interplay of light (stimulus) with the disperse system on a macroscopic scale, strongly support the theory. These findings enlarge the current view on self-oscillation phenomena and anticipate the colloidal state of matter to be a suitable host for accommodating light-propelled machineries. In broad terms, a complex system behavior is observed, which goes from periodic solutions (Hopf-Poincaré-Andronov bifurcation) to a new dynamic attractor driven by nanoparticle interactions, linking thermoplasmonics to nonlinearity and chaos.

Engineering metabolic cycle-inspired hydrogels with enzyme-fueled programmable transient volume changes

An enzyme-fueled transient volume phase transition (TVPT) of hydrogels under out-of-equilibrium conditions is reported. The approach takes inspiration from the metabolic cycle, comprising nutrient intake and anabolism/catabolism followed by waste excretion. The incorporation of methacrylic acid and acrylated trypsin in a polymeric hydrogel allowed the TVPT of the gel to be fueled by lysozyme. With the intake of lysozyme as fuel, the construction/destruction of electrostatic cross-linkages induced transient shrinkage/swelling of the gel accompanied by the depletion of lysozyme activity. The system's transient response could be flexibly programmed by adjusting not only the fuel concentration but the chemical composition of materials. The lysozyme-fueled TVPT of the gel could be exploited to transient changes in the mechanical properties of the gel. Our work opens a route toward a new class of stimuli-responsive hydrogels for biomedical applications.

DNA-Based Signaling Networks for Transient Colloidal Co-Assemblies

Programmable chemical circuits inspired by signaling networks in living cells are a promising approach for the development of adaptive and autonomous self-assembling molecular systems and material functions. Progress has been made at the molecular level, but connecting molecular control circuits to self-assembling larger elements such as colloids that enable real-space studies and access to functional materials is sparse and can suffer from kinetic traps, flocculation, or difficult system integration protocols. Herein, we report a toehold-mediated DNA strand displacement reaction network capable of autonomously directing two different microgels into transient and self-regulating co-assemblies. The microgels are functionalized with DNA and become elemental components of the network. The flexibility of the circuit design allows the installation of delay phases or accelerators by chaining additional circuit modules upstream or downstream of the core circuit. The design provides an adaptable and robust route to regulate other building blocks for advanced biomimetic functions.

Cyclic Macroscopic Assembly and Disassembly Driven by Ionic Strength Fuel: A Waste-Free Approach

Nonequilibrium assembling systems developed so far have relied on chemical fuels to drive the programmable pH cycles, redox reactions, and metastable bond formations. However, these methods often result in the unwanted accumulation of chemical waste. Herein, we present a novel strategy for achieving cyclic and waste-free nonequilibrium assembly and disassembly of macroscopic hydrogels, utilizing an ionic strength-mediated approach. Our strategy involves using ammonium carbonate as a chemical fuel to temporally regulate the attractions between oppositely charged hydrogels via ionic strength-controlled charge screening and hydrogel elasticity changes. This chemical fuel effectively mediates the assembly/disassembly processes and prevents waste accumulation, as ammonium carbonate can completely decompose into volatile chemical waste. The cyclic and reversible assembly process can be achieved without significant damping due to the self-clearance mechanism, as long as the chemical fuel is repeatedly supplied. This concept holds promise for creating macroscopic and microscopic nonequilibrium systems and self-adaptive materials.

Stimuli-Responsive Langmuir Films Composed of Nanoparticles Decorated with Poly(N-isopropyl acrylamide) (PNIPAM) at the Air/Water Interface

The nanotechnology shift from static toward stimuli-responsive systems is gaining momentum. We study adaptive and responsive Langmuir films at the air/water interface to facilitate the creation of two-dimensional (2D) complex systems. We verify the possibility of controlling the assembly of relatively large entities, i.e., nanoparticles with diameter around 90 nm, by inducing conformational changes within an about 5 nm poly(N-isopropyl acrylamide) (PNIPAM) capping layer. The system performs reversible switching between uniform and nonuniform states. The densely packed and uniform state is observed at a higher temperature, i.e., opposite to most phase transitions, where more ordered phases appear at lower temperatures. The induced nanoparticles' conformational changes result in different properties of the interfacial monolayer, including various types of aggregation. The analysis of surface pressure at different temperatures and upon temperature changes, surface potential measurements, surface rheology experiments, Brewster angle microscopy (BAM), and scanning electron microscopy (SEM) observations are accompanied by calculations to discuss the principles of the nanoparticles' self-assembly. Those findings provide guidelines for designing other adaptive 2D systems, such as programable membranes or optical interfacial devices.

Chemically Fueled Reinforcement of Polymer Hydrogels

Carbodiimide-fueled anhydride bond formation has been used to enhance the mechanical properties of permanently crosslinked polymer networks, giving materials that exhibit transitions from soft gels to covalently reinforced gels, eventually returning to the original soft gels. Temporary changes in mechanical properties result from a transient network of anhydride crosslinks, which eventually dissipate by hydrolysis. Over an order of magnitude increase in the storage modulus is possible through carbodiimide fueling. The time-dependent mechanical properties can be modulated by the concentration of carbodiimide, temperature, and primary chain architecture. Because the materials remain rheological solids, new material functions such as temporally controlled adhesion and rewritable spatial patterns of mechanical properties have been realized.

Active control of equilibrium, near-equilibrium, and far-from-equilibrium colloidal systems

The development of top-down active control over bottom-up colloidal assembly processes has the potential to produce materials, surfaces, and objects with applications in a wide range of fields spanning from computing to materials science to biomedical engineering. In this review, we summarize recent progress in the field using a taxonomy based on how active control is used to guide assembly. We find there are three distinct scenarios: (1) navigating kinetic pathways to reach a desirable equilibrium state, (2) the creation of a desirable metastable, kinetically trapped, or kinetically arrested state, and (3) the creation of a desirable far-from-equilibrium state through continuous energy input. We review seminal works within this framework, provide a summary of important application areas, and present a brief introduction to the fundamental concepts of control theory that are necessary for the soft materials community to understand this literature. In addition, we outline current and potential future applications of actively-controlled colloidal systems, and we highlight important open questions and future directions.

Adaptive 2D and Pseudo-2D Systems: Molecular, Polymeric, and Colloidal Building Blocks for Tailored Complexity

Two-dimensional and pseudo-2D systems come in various forms. Membranes separating protocells from the environment were necessary for life to occur. Later, compartmentalization allowed for the development of more complex cellular structures. Nowadays, 2D materials (e.g., graphene, molybdenum disulfide) are revolutionizing the smart materials industry. Surface engineering allows for novel functionalities, as only a limited number of bulk materials have the desired surface properties. This is realized via physical treatment (e.g., plasma treatment, rubbing), chemical modifications, thin film deposition (using both chemical and physical methods), doping and formulation of composites, or coating. However, artificial systems are usually static. Nature creates dynamic and responsive structures, which facilitates the formation of complex systems. The challenge of nanotechnology, physical chemistry, and materials science is to develop artificial adaptive systems. Dynamic 2D and pseudo-2D designs are needed for future developments of life-like materials and networked chemical systems in which the sequences of the stimuli would control the consecutive stages of the given process. This is crucial to achieving versatility, improved performance, energy efficiency, and sustainability. Here, we review the advancements in studies on adaptive, responsive, dynamic, and out-of-equilibrium 2D and pseudo-2D systems composed of molecules, polymers, and nano/microparticles.

Steering particles via micro-actuation of chemical gradients using model predictive control

Biological systems rely on chemical gradients to direct motion through both chemotaxis and signaling, but synthetic approaches for doing the same are still relatively naïve. Consequently, we present a novel method for using chemical gradients to manipulate the position and velocity of colloidal particles in a microfluidic device. Specifically, we show that a set of spatially localized chemical reactions that are sufficiently controllable can be used to steer colloidal particles via diffusiophoresis along an arbitrary trajectory. To accomplish this, we develop a control method for steering colloidal particles with chemical gradients using nonlinear model predictive control with a model based on the unsteady Green's function solution of the diffusion equation. We illustrate the effectiveness of our approach using Brownian dynamics simulations that steer single particles along paths, such as circle, square, and figure-eight. We subsequently compare our results with published techniques for steering colloids using electric fields, and we provide an analysis of the physical parameter space where our approach is useful. Based on these findings, we conclude that it is theoretically possible to explicitly steer particles via chemical gradients in a microfluidics paradigm.

Stochastic paths controlling speed and dissipation

Natural processes occur in a finite amount of time and dissipate energy, entropy, and matter. Near equilibrium, thermodynamic intuition suggests that fast irreversible processes will dissipate more energy and entropy than slow quasistatic processes connecting the same initial and final states. For small systems, recently discovered thermodynamic speed limits suggest that faster processes will dissipate more than slower processes. Here, we test the hypothesis that this relationship between speed and dissipation holds for stochastic paths far from equilibrium. To analyze stochastic paths on finite timescales, we derive an exact expression for the path probabilities of continuous-time Markov chains from the path summation solution to the master equation. We present a minimal model for a driven system in which relative energies of the initial and target states control the speed, and the nonequilibrium currents of a cycle control the dissipation. Although the hypothesis holds near equilibrium, we find that faster processes can dissipate less under far-from-equilibrium conditions because of strong currents. This model serves as a minimal prototype for designing kinetics to sculpt the nonequilibrium path space so that faster paths produce less dissipation.

Insights into Chemically Fueled Supramolecular Polymers

Supramolecular polymerization can be controlled in space and time by chemical fuels. A nonassembled monomer is activated by the fuel and subsequently self-assembles into a polymer. Deactivation of the molecule either in solution or inside the polymer leads to disassembly. Whereas biology has already mastered this approach, fully artificial examples have only appeared in the past decade. Here, we map the available literature examples into four distinct regimes depending on their activation/deactivation rates and the equivalents of deactivating fuel. We present increasingly complex mathematical models, first considering only the chemical activation/deactivation rates (i.e., transient activation) and later including the full details of the isodesmic or cooperative supramolecular processes (i.e., transient self-assembly). We finish by showing that sustained oscillations are possible in chemically fueled cooperative supramolecular polymerization and provide mechanistic insights. We hope our models encourage the quantification of activation, deactivation, assembly, and disassembly kinetics in future studies.

Assembling Inorganic Nanocrystal Gels

Inorganic nanocrystal gels retain distinct properties of individual nanocrystals while offering tunable, network-structure-dependent characteristics. We review different mechanisms for assembling gels from colloidal nanocrystals including (1) controlled destabilization, (2) direct bridging, (3) depletion, as well as linking mediated by (4) coordination bonding or (5) dynamic covalent bonding, and we highlight how each impacts gel properties. These approaches use nanocrystal surface chemistry or the addition of small molecules to mediate inter-nanocrystal attractions. Each method offers advantages in terms of gel stability, reversibility, or tunability and presents new opportunities for the design of reconfigurable materials and fueled assemblies.

Nanoparticle Assembly as a Materials Development Tool

Nanoparticle assembly is a complex and versatile method of generating new materials, capable of using thousands of different combinations of particle size, shape, composition, and ligand chemistry to generate a library of unique structures. Here, a history of particle self-assembly as a strategy for materials discovery is presented, focusing on key advances in both synthesis and measurement of emergent properties to describe the current state of the field. Several key challenges for further advancement of nanoparticle assembly are also outlined, establishing a roadmap of critical research areas to enable the next generation of nanoparticle-based materials synthesis.

Nonequilibrium Catalytic Supramolecular Assemblies of Melamine- and Imidazole-Based Dynamic Building Blocks

The development of synthetic nonequilibrium systems has gathered increasing attention due to their potential to illustrate the dynamic, complex, and emergent traits of biological systems. Simple building blocks capable of interacting via dynamic covalent chemistry and physical assembly in a reaction network under nonequilibrium conditions can contribute to our understanding of complex systems of life and its origin. Herein, we have demonstrated the nonequilibrium generation of catalytic supramolecular assemblies from simple heterocycle melamine driven by a thermodynamically activated ester. Utilizing a reversible covalent linkage, an imidazole moiety was recruited by the assemblies to access a catalytic transient state that dissipated energy via accelerated hydrolysis of the activated ester. The nonequilibrium assemblies were further capable of temporally binding to a hydrophobic guest to modulate its photophysical properties. Notably, the presence of an exogenous aromatic base augmented the lifetime of the catalytic microphases, reflecting their higher kinetic stability.

Emergence of a Promiscuous Peroxidase Under Non-Equilibrium Conditions

Herein, we report the substrate induced generation of a transient catalytic microenvironment from a single amino acid functionalized fatty acid in presence of a cofactor hemin. The catalytic state accessed under non-equilibrium conditions showed acceleration of peroxidase activity resulting in degradation of the substrate and subsequently led to disassembly. Equilibrated systems could not access the three-dimensional microphases and showed substantially lower catalytic activity. Further, the assembled state showed latent catalytic function (promiscuity) to hydrolyze a precursor to yield the same substrate. Consequently, the assembly demonstrated protometabolism by exploiting the peroxidase-hydrolase cascade to augment the lifetime and the mechanical properties of the catalytic state.

Polymorphic transient glycolipid assemblies with tunable lifespan and cargo release

HYPOTHESIS

In living systems, dynamic processes like dissipative assembly, polymorph formation, and destabilization of hydrophobic domains play an indispensable role in the biochemical processes. Adaptation of biological self-assembly processes to an amphiphilic molecule leads to the fabrication of intelligent biomaterials with life-like behavior.

EXPERIMENTS

An amphiphilic glycolipid molecule was engineered into various dissipative assemblies (vesicles and supramolecular nanotube-composed hydrogels) by using two activation steps, including heating-cooling and shear force in method-1 or boric acid/glycolipid complexation and shear force in method-2. The influence of number of activation steps on vesicle to nanotube phase transitions and activation method on the properties of hydrogels were investigated, where the morphological transformations and destabilization of hydrophobic domains resulted from a bilayer to a higher-order crystal structure.

FINDINGS

Hydrophobic and hydrophilic cargos encapsulated in the dissipative assemblies (vesicles and injectable hydrogels) can be released in a controlled manner via changing the activation method. The reported adaptive materials engineered by dual activation steps are promising self-assembled systems for programmed release of loaded cargos at a tunable rate.

Reversible and spatiotemporal control of colloidal structure formation

Tuning colloidal structure formation is a powerful approach to building functional materials, as a wide range of optical and viscoelastic properties can be accessed by the choice of individual building blocks and their interactions. Precise control is achieved by DNA specificity, depletion forces, or geometric constraints and results in a variety of complex structures. Due to the lack of control and reversibility of the interactions, an autonomous oscillating system on a mesoscale without external driving was not feasible until now. Here, we show that tunable DNA reaction circuits controlling linker strand concentrations can drive the dynamic and fully reversible assembly of DNA-functionalized micron-sized particles. The versatility of this approach is demonstrated by programming colloidal interactions in sequential and spatial order to obtain an oscillatory structure formation process on a mesoscopic scale. The experimental results represent an approach for the development of active materials by using DNA reaction networks to scale up the dynamic control of colloidal self-organization.

Transient Dissipative Optical Properties of Aggregated Au Nanoparticles, CdSe/ZnS Quantum Dots, and Supramolecular Nucleic Acid-Stabilized Ag Nanoclusters

Transient, dissipative, aggregation and deaggregation of Au nanoparticles (NPs) or semiconductor quantum dots (QDs) leading to control over their transient optical properties are introduced. The systems consist of nucleic acid-modified pairs of Au NPs or pairs of CdSe/ZnS QDs, an auxiliary duplex L₁/T₁, and the nicking enzyme Nt.BbvCI as functional modules yielding transient aggregation/deaggregation of the NPs and dynamically controlling over their optical properties. In the presence of a fuel strand L₁', the duplex L₁/T₁ is separated, leading to the release of T₁ and the formation of duplex L₁/L₁'. The released T₁ leads to aggregation of the Au NPs or to the T₁-induced G-quadruplex bridged aggregated CdSe/ZnS QDs. Biocatalytic nicking of the L₁/L₁' duplex fragments L₁' and the released L₁ displaces T₁ bridging the aggregated NPs or QDs, resulting in the dynamic recovery of the original NPs or QDs modules. The dynamic aggregation/deaggregation of the Au NPs is followed by the transient interparticle plasmon coupling spectral changes. The dynamic aggregation/deaggregation of the CdSe/ZnS QDs is probed by following the transient chemiluminescence generated by the hemin/G-quadruplexes bridging the QDs and by the accompanying transient chemiluminescence resonance energy transfer proceeding in the dynamically formed QDs aggregates. A third system demonstrating transient, dissipative, luminescence properties of a reaction module consisting of nucleic acid-stabilized Ag nanoclusters (NCs) is introduced. Transient dynamic formation and depletion of the supramolecular luminescent Ag NCs system via strand displacement accompanied by a nicking process are demonstrated.

Exploiting Additives for Directing the Adsorption and Organization of Colloid Particles at Fluid Interfaces

The self-assembly of colloids at fluid interfaces is a well-studied research field both for gaining fundamental insights and for material fabrication. The fluid interface allows the confinement of particles in two dimensions and may act as a template for guiding their organization into soft and reconfigurable structures. Additives (e.g., surfactants, salts, and polymers) in the colloidal suspension are routinely used as a practical and effective tool to drive particle adsorption and tune their interfacial organization. However, some phenomena lying at the heart of the accumulation and self-assembly of particles at fluid interfaces remain poorly understood. This Feature Article aims to critically analyze the mechanisms involved in the adsorption and self-organization of micro- and nanoparticles at various fluid interfaces. In particular, we address the role of additives in both promoting the adsorption of particles from the bulk suspension to the fluid interface and in mediating the interactions between interfacial particles. We emphasize how different types of additives play a crucial role in controlling the interactions between suspended particles and the fluid interface as well as the interactions between adsorbed particles, thus dictating the final self-assembled structure. We also critically summarize the main experimental protocols developed for the complete adsorption of particles initially suspended in the bulk. Furthermore, we highlight some special properties (e.g., reconfigurability upon external stimulation and dissipative self-assembly) and the application potential of structures formed by colloid self-organization at fluid interfaces mediated/promoted by additives. We believe our contribution serves both as a practical roadmap to scientists coming from other fields and as a valuable information resource for all researchers interested in this exciting research field.

Signaling in Systems Chemistry: Programing Gold Nanoparticles Formation and Assembly Using a Dynamic Bistable Network

Living cells exploit bistable and oscillatory behaviors as memory mechanisms, facilitating the integration of transient stimuli into sustained molecular responses that control downstream functions. Synthetic bistable networks have also been studied as memory entities, but have rarely been utilized to control orthogonal functions in coupled dynamic systems. We herein present a new cascade pathway, for which we have exploited a well-characterized switchable peptide-based replicating network, operating far from equilibrium, that yields two alternative steady-state outputs, which in turn serve as the input signals for consecutive processes that regulate various features of Au nanoparticle shape and assembly. This study further sheds light on how bridging together the fields of systems chemistry and nanotechnology may open up new opportunities for the dynamically controlled design of functional materials.

The Transient Covalent Bond in Abiotic Nonequilibrium Systems

Biochemical systems accomplish many critical functions with by operating out-of-equilibrium using the energy of chemical fuels. The formation of a transient covalent bond is a simple but very effective tool in designing analogous reaction networks. This Minireview focuses on the fuel chemistries that have been used to generate transient bonds in recent demonstrations of abiotic nonequilibrium systems (i.e., systems that do not make use of biological components). Fuel reactions are divided into two fundamental classifications depending on whether the fuel contributes structural elements to the activated state, a distinction that dictates how they can be used. Reported systems are further categorized by overall fuel reaction (e.g., hydrolysis of alkylating agents, carbodiimide hydration) and illustrate how similar chemistry can be used to effect a wide range of nonequilibrium behavior, ranging from self-assembly to the operation of molecular machines.

Colloidal Nanocrystal Gels from Thermodynamic Principles

Gels assembled from solvent-dispersed nanocrystals are of interest for functional materials because they promise the opportunity to retain distinctive properties of individual nanocrystals combined with tunable, structure-dependent collective behavior. By incorporating stimuli-responsive components, these materials could also be dynamically reconfigured between structurally distinct states. However, nanocrystal gels have so far been formed mostly through irreversible aggregation, which has limited the realization of these possibilities. Meanwhile, gelation strategies for larger colloidal microparticles have been developed using reversible physical or chemical interactions. These approaches have enabled the experimental navigation of theoretically predicted phase diagrams, helping to establish an understanding of how thermodynamic behavior can guide gel formation in these materials. However, the translation of these principles to the nanoscale poses both practical and fundamental challenges. The molecules guiding assembly can no longer be safely assumed to be vanishingly small compared to the particles nor large compared to the solvent.In this Account, we discuss recent progress toward the assembly of tunable nanocrystal gels using two strategies guided by equilibrium considerations: (1) reversible chemical bonding between functionalized nanocrystals and difunctional linker molecules and (2) nonspecific, polymer-induced depletion attractions. The effective nanocrystal attractions, mediated in both approaches by a secondary molecule, compete against stabilizing repulsions to promote reversible assembly. The structure and properties of the nanocrystal gels are controlled microscopically by the design of the secondary molecule and macroscopically by its concentration. This mode of control is compelling because it largely decouples nanocrystal synthesis and functionalization from the design of interactions that drive assembly. Statistical thermodynamic theory and computer simulation have been applied to simple models that describe the bonding motifs in these assembling systems, furnish predictions for conditions under which gelation is likely to occur, and suggest strategies for tuning and disassembling the gel networks. Insights from these models have guided experimental realizations of reversible gels with optical properties in the infrared range that are sensitive to the gel structure. This process avoids time-consuming and costly trial-and-error experimental investigations to accelerate the development of nanocrystal gel assemblies.These advances highlight the need to better understand interactions between nanocrystals, how interactions give rise to gel structure, and properties that emerge. Such an understanding could suggest new approaches for creating stimuli-responsive and dissipative assembled materials whose properties are tunable on demand through directed reconfiguration of the underlying gel microstructure. It may also make nanocrystal gels amenable to computationally guided design using inverse methods to rapidly optimize experimental parameters for targeted functionalities.